The amount of acoustic energy output of a speaker being fed back into a microphone of a duplex acoustic system with gain (i.e., a device used for communication purposes) determines the system acoustic stability. Such stability is important to prevent the generation of “singing” in which feedback of the speaker output onto the microphone causes reinforcement of sound from the loudspeaker and thus causes the speaker to emit a howl or similar high-pitched noise.
There are existing methods of preventing this singing effect that operate by inserting switched loss into either the speaker or microphone path to ensure system stability. The amount of switched loss to insert is determined by comparing the microphone signal level to the speaker signal level from the network via a hybrid connected to the speakerphone. Examination of the relative levels of the two signals permits a determination as to which signal level is presently active (i.e. speaker output or voice input). Loss is inserted in the path which is determined to be presently inactive ensuring that the total loop electroacoustic gain of the speakerphone and the network is less than one at the frequency where at zero degrees loop phase shift is experienced. This criterion, known as the Nyquist stability criterion, determines how much loss must be present in the electro-acoustic loop consisting of the speakerphone and the network to sustain oscillations, in order to ensure stability. The overall loss inserted, in many arrangements, to maintain stability is related to the sum of signal-dependent switched loss and some fixed loss amount, which is needed to provide “sing” margin to compensate for inaccuracies in determination of the total amount of loop gain necessary to prevent oscillations at specific frequencies.
The difficulty of these implementations has been in determining the amount of coupling which exists between the speakerphone's speaker and its microphone (i.e., speaker output vs. voice input). The acoustic environment between speaker and microphone is often unstable making a determination between speaker feedback and voice input to the microphone difficult to assess. In another arrangement, it has been thought possible to have the relative signal levels determined at the hybrid connection of the speakerphone to the telephone network. It is theoretically possible to sample incoming and outgoing speech at the hybrid connecting the phone to the network to infer loop gain, but this method has difficulties due to the isolation loss of the hybrid and is often unsatisfactory